In most current wireless communication systems, nodes in a network may be configured to operate based on a single transmit and a single receive antenna. However, for many current wireless systems, the use of multiple transmit and/or receive antennas may result in an improved overall system performance. These multi-antenna configurations, also known as smart antenna techniques, may be utilized to reduce the negative effects of multipath and/or signal interference may have on signal reception. Existing systems and/or systems which are being currently deployed, for example, CDMA-based systems, TDMA-based systems, WLAN systems, and OFDM-based systems such as IEEE 802.11 a/g/n, may benefit from configurations based on multiple transmit and/or receive antennas. It is anticipated that smart antenna techniques may be increasingly utilized both in connection with the deployment of base station infrastructure and mobile subscriber units in cellular systems to address the increasing capacity demands being placed on those systems. These demands arise, in part, from a shift underway from current voice-based services to next-generation wireless multimedia services that provide voice, video, and data communication.
The utilization of multiple transmit and/or receive antennas is designed to introduce a diversity gain and array gain and to suppress interference generated within the signal reception process. Such diversity gains improve system performance by increasing received signal-to-noise ratio, by providing more robustness against signal interference, and/or by permitting greater frequency reuse for higher capacity. Systems that utilize multiple transmit and multiple receive antenna may be referred to as multiple-input multiple-output (MIMO) systems. One attractive aspect of multi-antenna systems, in particular MIMO systems, is the significant increase in system capacity that may be achieved by utilizing these transmission configurations. For a fixed overall transmitted power, the capacity offered by a MIMO configuration may scale with the increased signal-to-noise ratio (SNR).
In order to transfer maximum energy or power between a transmit and a receive antenna, both antennas should have the same spatial orientation, the same polarization sense and the same axial ratio. When the antennas are not aligned or do not have the same polarization, there may be a reduction in energy or power transfer between the two antennas. This reduction in power transfer may reduce the overall system efficiency and performance. When the transmit and receive antennas are both linearly polarized, physical antenna misalignment may result in a polarization mismatch loss.
Multipath signals may arrive at a mobile handset antenna via the reflection of the direct signal off of nearby objects. If the reflecting objects are oriented such that they are not aligned with the polarization of the incident wave, the reflected wave may experience a shift in polarization shift. The resultant or total signal available to the receiver at either end of the communications link may be a vector summation of the direct signal and all of the multipath signals. In many instances, there may be a number of signals arriving at the receive site that are not aligned with the polarization of the system antenna. As the receive antenna rotates from vertical to horizontal, it may intercept or receive energy from these multiple signals.
In polarization diversity systems, a dual linear polarized antenna may be utilized to receive samples and track the polarization output providing the strongest signal level. Each output may provide a total signal that may be a combination of all incident signals. This combined signal may be a function of the amplitude and phase of each signal as well as the polarization mismatch of each signal.
Transmit antenna diversity may be utilized to obtain diversity gain against Rayleigh fading in wireless systems where the mobile user has a limited number of antennas. Antenna hopping may utilize transmit antennas to obtain diversity gain. With antenna hopping, cyclic or pseudo-random hopping may be used to transform spatial diversity into time diversity, which may be exploited by, appropriate error correction codes and interleaving techniques, but the interleaving requirements and/or possible bandwidth expansion may incur latency due to the error correction code.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.